1. Field of the Invention
This invention relates to a display panel, especially to a bidirectional shift register circuit used in a gate driving circuit of a display panel.
2. Description of the Prior Art
In recent years, with the continuous progress of TFT-LCD panel technology, driving circuits can be integrated on the panel through the system on glass (SOG) technology.
For example, the driving circuits integrated on the panel can be formed by the amorphous silicon (a-Si) manufacturing process or low temperature poly-silicon (LTPS) manufacturing process, and the largest differences between them are electrical properties and manufacturing process complexity. Although the LTPS thin-film transistor has higher carrier mobility, its manufacturing process is quite complicated. The a-Si thin-film transistor has poor carrier mobility, but its manufacturing process is simpler and mature; therefore, the a-Si thin-film transistor has advantages of cost.
However, the threshold voltage of the a-Si thin-film transistor will be affected by a bias and rise gradually due to the limitations of process capability; therefore, the SOG technology fails to be achieved.
Please refer to FIG. 1. FIG. 1 illustrates a schematic diagram of the conventional bidirectional shift register circuit. As shown in FIG. 1, in the conventional bidirectional shift register circuit 1, the gate driving circuit 15 needs two same discharging circuits 11 and 12 to discharge in turn to stabilize the desired voltage level. Similarly, the gate driving circuit 16 also needs two same discharging circuits 13 and 14 to discharge in turn to stabilize the desired voltage level, and so on.
It is assumed that the conventional bidirectional shift register circuit 1 is formed by a-Si thin-film transistors, if the threshold voltages of some a-Si thin-film transistors are shifted, with increasing of using time, the operation of the conventional bidirectional shift register circuit 1 would be seriously affected by the threshold voltage shift of these a-Si thin-film transistors, even the conventional bidirectional shift register circuit 1 would be failed.
In addition, because the designs of ultra high definition and slim border are required in current display panel of the high-end mobile phones and the tablet PC, dummy gate drivers should be additionally disposed in the conventional bidirectional shift register circuit 1, and it will cause poor reliability issue of the conventional bidirectional shift register circuit 1.